There are a number of different timing systems and devices, generally referred to as time-temperature indicators (TTIs), which can be used to monitor the exposure of objects to a range of temperatures over a specified period of time. In an early example, Witonsky, in U.S. Pat. No. 3,942,467, describes a time-temperature indicator with an encapsulated inner container and a pH sensitive dye solution contained therein. The device of Witonsky further has an encapsulated outer container containing an organic material which undergoes solvolysis. The outer container and the inner container are separated by a membrane. When the membrane between the inner container and the outer container is broken, the contents of the containers mix and over a period of time change color, thus providing an indication of the passage of a duration of time. A number of other time-temperature indicators utilize wicking techniques (such as described in U.S. Pat. No. 5,709,472 and U.S. Pat. No. 6,042,264, both issued to Prusik et al.) or diffusion layer techniques (such as described in U.S. Pat. No. 4,629,330 issued to Nichols and U.S. Pat. Nos. 5,930,206 and 5,633,835 both issued to Haas et al.). In U.S. Pat. No. 6,198,701 issued to De Jonghe et al., an electrochemical timing device is described, whereby consumption of an electrode is used to provide an indication of the passage of a duration of time.
Time-temperature indicators can have a number of different applications for indicating when an event or activity needs to take place. For example, time-temperature indicators have applications for indicating when the perishable materials have expired and need to be thrown out. Time-temperature indicators also have applications for general inventory management, for monitoring projects, activities and a host of other time and/or temperature dependent events. Therefore, there is a continued need to develop reliable timing systems and devices which can be used for a variety of different applications.
In a further aspect, time-temperature indicators may be used to indicate a level or time of exposure to environmental attributes. Particularly, such a timing device is adaptable for indicating a time and/or level of exposure to harmful environmental attributes including ultraviolet radiation, x-ray radiation, nuclear radiation, biological contamination and physical contamination, to name a few. In this respect, the timing device has applications for indicating when a safe time or exposure threshold has been reached.